Various apparatus and methods have been proposed for supplementing heat applied to water in a water heater tank by means of a heat pump that acquires heat from air ambient to the water heater and conveys the acquired heat to the water tank water via a heat exchanger.
In a prior art system illustrated in FIG. 1A, for example, a water heater 10 comprises a tank 12 formed by a metal, for example steel, polymer, or porcelain tank wall that encloses a volume of water therein and that is, in turn, enclosed by an outer metal housing 18. Tank 12 receives cold water from a cold water inlet 14 and expels hot water from a hot water outlet 16. Two heating elements (not shown) are secured within harnesses (not shown) attached to and extending through outer housing 18 and that extend through and attach to the outer surface of tank 12. Each heating element attaches to a respective harness and extends through the wall of tank 12 into the tank's interior volume. An electrical power source provides electric current to each heating element under the control of the water heater's control system so that the electric current passes through the resistive elements, causing their temperature to rise and thereby causing the resistive elements to contribute heat to water within the tank interior volume. The control system actuates the resistive heating elements (i.e., provides power to them) in response to the output of one or more temperature sensors attached to the exterior of tank 12 or extending therethrough that provide signals to the control system indicating the temperature of water within the tank volume. In particular, the control system actuates the heating elements when the tank water temperature is low and deactivates the one or more heating elements when the tank water temperature reaches a predetermined upper set point.
Cold water from inlet 14 is attached to a private or public water system that provides water under pressure to end user water systems such as water heater 10. Hot water outlet 16 is attached to a hot water piping system within a residential or commercial building that delivers hot water to faucets, appliances, and other equipment that draw hot water upon actuation of an associated valve. When those valves are open, causing low pressure at hot water outlet 16, water pressure within tank 12 (maintained by pressure applied by the water source at cold water inlet 14) expels heated water through outlet 16.
A refrigerant conduit 20 conducts refrigerant through a refrigerant path that encompasses a condenser coil portion 22, an expansion valve 24, an evaporator coil 26, and a compressor 28. Condenser coil 22 comprises a portion of refrigerant conduit 20 that wraps around the exterior of tank 12, inside the enclosure of outer tank housing 18. Following condenser coil 22, refrigerant conduit 20 leads to expansion valve 24. As should be understood, the expansion valve receives a fluid input at a high pressure and, depending on the settings within the valve, outputs the fluid at a lower pressure, allowing the pressurized refrigerant entering the valve to drop in pressure in the coil of evaporator 26 and change phase from a liquid to a gas. As should also be understood, compressor 28 is a pump that additionally provides pressure to refrigerant flowing through the refrigerant path to thereby maintain the refrigerant flowing through the complete closed loop that the path defines.
More specifically, compressor 28 pumps the gaseous refrigerant received from evaporator 26 forward, increasing the refrigerant's pressure and temperature and causing the now-hotter refrigerant gas to flow through condenser coil 22. The hot refrigerant is separated from water within tank 12 by the refrigerant conduit line wall and the wall of tank 12, both of which may be metallic and therefore relatively heat-conductive. Thus, as the refrigerant travels through the length of condenser coil 22, the refrigerant transfers heat through these walls to the cooler water within the inner tank volume. The refrigerant thereby acts as a heat source that supplements the resistive heating elements.
As refrigerant flows through condenser coil 22, it changes phase from gas to liquid. Still under the pressure provided by compressor 28, however, the now-liquid refrigerant flows from condenser 22 to expansion valve 24, which drops the liquid refrigerant's pressure as it enters evaporator coil 26. A fan 30 is actuated concurrently with compressor 28 and is positioned adjacent holes in housing 18 so that the fan pushes an output air stream 32 from a volume 34 within the upper portion of housing 18, across evaporator coil 26, through the holes, and out to an exterior area ambient to the water tank. Outer housing 18 defines a second set of holes 36 on the opposite side of volume 34 from the holes adjacent to fan 30 and evaporator 26, so that fan 30 also draws an input air stream 38 into volume 34. Thus, fan 30 draws an air flow from outside water heater 10, into volume 34, and across compressor 28, through evaporator coil 26, and out of water heater 10 at air flow 32. Particularly where water heater 10 is in a building, ambient air 38 is at a relatively warm temperature, but as the air flow passes over compressor 28 during the compressor's operation, the air flow draws further heat generated by the compressor. Within evaporator 26, the now-lower pressure refrigerant draws heat energy from the air flow over coil 26 and transitions to a gaseous phase. The now-warmer gaseous refrigerant discharged from evaporator coil 26 then returns to compressor 28 via a suction portion 40 of refrigerant line 20, and the now-cooler air flow 32 flows out of the water heater housing through the holes in the housing in front of the evaporator fan, and the cycle repeats.
As is apparent from the discussion above regarding water tank 10 as illustrated in FIG. 1A, condenser 22 forms part of a heat exchanger that transfers heat between the refrigerant of conduit line 20 and the water stored in the inner volume of tank 12. In a prior art configuration illustrated in FIG. 1B, condenser 22 is part of a heat exchanger that is separate from tank 12. In this arrangement, tank 12, compressor 28, evaporator 26, fan 30, the air flow, and conduit line 20 operate as discussed above with respect to FIG. 1A, except that the portion of conduit line 20 forming condenser coil 22 does not wrap around the exterior of tank 12. Instead, coil 22 is housed in a middle chamber 42 disposed between upper volume 34 and the lower volume that encloses tank 12. A water line 42 extends from the inner volume of tank 12 to and from a heat exchanger in which condenser coil 22 is also disposed. A pump (not shown) is provided in line 42 to pump the tank water to and from the heat exchanger. The refrigerant line of coil 22 and the water line of coil 42 are adjacent to one another in the heat exchanger, so that the refrigerant flowing through coil 22 contributes heat to the water flowing through line 42 across the walls of conduit 20 and conduit 42. Otherwise, the system illustrated in FIG. 1B operates in a manner as does the system illustrated in FIG. 1A.
Because air flow 32 exiting the housing has been cooled by its passage over the evaporator, attempts have been made to attach ducts of building heating, ventilation, and air-conditioning (HVAC) systems to the flat side of the water heater at the outlet, to thereby acquire the cooled air for contribution to the building's air-conditioned space. Because the duct introduces resistance to air flow, however, this practice increases the flow resistance seen by the air flow generated by the evaporator fan, thereby increasing an amount of that air flow that, instead of flowing out of the water heater housing and into the duct, flows radially (with respect to the forward air flow direction away from the fan) away from the fan but still within the water heater housing. This can, in turn, increase pressure in the water heater's upper chamber, thereby lowering the temperature in the upper chamber and decreasing the fan's ability to draw warmer air from outside the water heater into the air flow, in turn thereby lowering the water heater's efficiency.
In particular, it was known to attach a flange on the water heater exterior outward of the outlet orifice so that the duct could be attached to the flange. Such an arrangement required effort on the part of the retrofitter to attach the flange, and the retrofit configuration could introduce a pressure drop. When a fluid leaves an orifice into a space having a cross-sectional area greater than that of the orifice (e.g., in the direction of air flow, the cross-sectional area has a step-increase from the orifice into the space), the diverging air flow streamlines and recirculating flow immediately downstream of the orifice may cause a pressure drop that increases flow resistance. In retrofitting, due to the difficulty of sealing a duct over the orifice, the ductwork is oversized compared to the cross-sectional area of the hole(s) to thereby fully cover the outlet, in turn forming an orifice with such a pressure drop.
Other heat exchange arrangements are possible, for example as discussed at A. Hepbasli and Y. Kalinci, A Review of Heat Pump Water Heating Systems, Renew. Sustain. Energy Rev. (2008).
If shipped on their sides, hybrid water heaters may be subject to damage for a number of reasons. First, it is generally known that if oil leaks out of a compressor through its discharge tube and does not return to the compressor in the reverse direction, the leaked oil may in some circumstances cause the compressor to fail due to lack of lubrication. Second, the compressor typically “floats” on isolation pads in order to dampen vibrations and minimize noise. That is, the compressor mounts to the tank via non-rigid couplings. Therefore, the compressor, when suspended so that it is cantilevered and extends sideways from its mount, can be subject to considerable movement. This may cause stresses in the tubing of the refrigerant conduit line, which is generally rigid, and further creates a risk that the compressor will impact the evaporator coil, thereby damaging the evaporator coil or its heat exchanger fins and decreasing performance.